1. Field of the Invention
This invention pertains to interferometric metrology of optical surfaces and, more specifically, to the precise measurement of reflective optical surfaces as used in lasers and reflective telescopes.
2. Description of the Prior Art
Three types of interferometers are commonly used as surface measuring instruments: the Twyman-Green Interferometer (F. Twyman, A. Green, British Pat. No. 103,832, 1916); Shearing Interferometer (W. J. Bates, Proc.Phys.Soc.London, 59,940,1947) and Fizeau Interferometer (H. Fizeau, Ann.Chim. Phys., 3, 66, 1962, 429). All three interferometers require high-quality input wavefronts and with the exception of the shearing interferometer, high quality reference surfaces are necessary. By multiple exposure interferograms and three nearly identical reference surfaces the achievable precision in the surface measurement approaches .lambda./100 (G. D. Dew: The Measurement of Optical Flats, J. Sci. Instr. 1966, Vol. 43).
New electronic readout schemes implemented into these interferometers allow for high speed interpretation of the resulting fringe patterns, e.g., heterodyne interferometry (J. A. Wyant: Use of Heterodyne Interferometry in Optical Testing, JOSA 1974, 1363).
One general constraint of all interferometers is the requirement that the interfering wavefronts must be spherical. Consequently, the testing of aspherical surfaces requires auxiliary optics which converts the aspherical wavefront into a spherical one (A. Offner, A. Null Corrector for Paraboliodal Mirrors, Appl. Optics, February 1963, 153). Recently, holography has been applied successfully to replace the auxiliary optics in interferometry of aspherical optical surfaces (P. M. Emmel et al: Holographic and Shearing Interfer. for Optical Testing Including Aspherics, JOSA 1978, 1416); since the required shape of the aspherics is known, the necessary hologram can be generated by a computer. Unfortunately, the computer generated holograms required to test steep aspherics are also difficult to produce (J. C. Wyant et al, Computer Generated Holograms; Null Lens Test of Aspheric Wavefronts, Appl. Opt. December 1974, 2762).
A high resolution non-interferometric surface measurement scheme has recently been reported (A. E. Ennos: High Accuracy Profile Measurement of Quasi-Conical Mirror Surfaces by Laser Auto-Collimation, Precision Engineering, Vol.4, #1, January 1982). Here the slope of the surface-reflected laser pencil beam is measured with high accuracy via high precision readout electronics. The accuracy of the surface profile measurement is reported to be .+-.2 nm.
The Pencil Beam Interferometer is a variation of the Fan Trace Interferometer (K. von Bieren, Wavefront Investigation of a Fourier Transform Lens which the Fan Trace Interferometer, Appl. Opt. July 1973, 1642) which has been adapted to measure reflective surface profiles to a high degree of accuracy. Unlike the previously-mentioned interferometers, it requires neither a high-quality, large-aperture input wave to measure large aperture optics nor a reference wavefront. Hence, the pencil beam interferometer is suitable for the measurement of surface profiles of spherical as well as aspherical optics.